It is known to employ feedback and feedforward control systems to control the height of a probe in a scanning probe microscope. See for example Pao, L. Y., Butterworth, J. A., and Abramovitch, D. Y., 2007, “Combined Feedforward/Feedback Control of Atomic Force Microscopes,” Proceedings of the 2007 American Control Conference, New York, N.Y., July 11-13, pp. 3509-3515, and also Clayton et al., 2009, “A Review of Feedforward Control Approaches in Nanopositioning for High-Speed SPM”, Journal of Dynamic Systems, Measurement, and Control, 131:6, 061101.
In such conventional control systems, the height of the probe is controlled by a piezo actuator and the angle of the probe is measured by an optical sensor which generates a detection signal. The feedback system receives as inputs the detection signal and a desired value known as a set point. The feedback system then controls the piezo actuator so that the detection signal remains at the set point as a sample is scanned.
EP-A-1898204 describes a scanning probe microscope with a laser which controls displacement of a cantilever. An amplitude detecting circuit determines an amplitude of the cantilever to generate a detected amplitude signal. A feedback circuit subtracts the detected amplitude signal from a reference amplitude signal to generate a deviation signal and a feedback signal. The amplitude of the cantilever is kept constant by the feedback circuit. Thus EP-A-1898204 discloses a probe actuation system according to the pre-amble of claim 1.
WO-A-2012/104625 describes an adaptive mode scanning probe microscope in which the probe is moved repeatedly towards and away from the sample surface, only contacting it intermittently.